Pressure pad for riveting machines



Jan. 30, 1951 G. T. HARCOURT [PRESSURE PAD FOR RIVETING MACHINES r W l w a f H e .N m i w n w .6 A mm W. 5 em mm 8 M. l u w M d e 1 .Fl

Jan. 30, 1951 G. 'r. HARCOURT PRESSURE PAD FOR RIVETING MACHINES 3 Sheets-Sheet 2 Filed May 11, 1948 lNVENTOR 550%.; f/fil/Pcozwz ATTORNEY Jan. 30, 1951 G. T. HARCOURT PRESSURE PAD FOR RIVETING MACHINES 3 Sheets-Sheet 5 Filed May 11, 1948 INVENTO R 'zawas 7." HA/Pmu/FI ATTO RN EY Patented Jan. 30, 1951 UNITED STATES FATENT OFFICE PRESSURE PAD FOR RIVETING MACHINES George T. Harcourt, Detroit, Mich.

Application May 11, 1948, Serial No. 26,296

6 Claims. 1

This invention relates to portable riveting tools and more particularly to a portable hydraulic riveter which is especially useful for seam cap riveting operations.

Seam cap riveting is illustrated for example in the United States Patent 2,085,540, issued June 29, 1937, to Walter P. Murphy, and which concerns a roof construction for metal freight cars. In seam cap riveting it is essential that the folds of metal which comprise the seam cap assembly, must be securely clamped prior to and during the riveting operation if a good rivet setting is to be achieved. In the Murphy car roof seam cap little space is afforded for clamping, and in addition thereto, brackets for the car running board which are sometimes riveted with the seam cap, further complicates the riveting operation.

It is the object of this invention to provide a portable hydraulic riveter which can be conveniently used by an operator to eiiiciently rivet a seam cap such as is incorporated in a Murphy car roof construction.

A further object is to provide a portable hydraulic riveter having a universal type suspension means whereby the riveter can be easily and quickly positioned for riveting operations.

Another object is to provide a hydraulic riveter having an adjustable pad for clamping the work prior to and during the rivet setting operation. A feature of the invention is a toggle link arrangement interposed between the pressure pad and yoke and so arranged that the reaction of the yoke to the pressure against the rivet set is at first transmitted entirely to the pressure pad to clamp the work pieces firmly together, and

later with a decreasing component of reactive force taken by the pad with a correspondingly increasing portion taken by the dolly to head the rivet.

Further objects of the invention are to provide a hydraulic riveter which is compact in structure, reliable in operation, easy to manipulate and which is extremely serviceable and has low maintenance costs.

These and further objects and features of the invention will become apparent when viewed in the light of the following specification and accompanying drawings wherein:

Fig. 1 is a side elevation, partly in section, of a portable hydraulic riveter, according to one embodiment of the invention;

Fig. 2 is a fragmentary longitudinal section of L the slotted rear end of the ram or rivet set holder,

2 showing in broken lines a part of the rocket arm which actuates the holder;

Fig. 3 is a perspective view of the pressure pad;

Fig. 4 is a front end view of the riveter shown in Fig. 1, as seen from the left thereof;

Fig. 5 is a cross section along the irregular line 5-5 in Fig. 1;

Fig. 6 is a bottom View, partly in section, of the pressure pad portion of the tool shown in Fig. 1;

Fig. 7 is a longitudinal section of a portion of the riveter as seen along line l'l in Fig. 1, looking upward;

Fig. 8 is a cross section through the depending portion of the yoke and the rear toggle links, as indicated by the arrows 8 in Fig. 6;

Fig. 9 is a cross section through the depending portion of the yoke, the front toggle links and the pressure pad extensions as indicated by the arrows 9 in Fig. 6;

Fig. 10 is a bottom plan view of a modified riveter, showing the parts of the tool in their normal or inoperative position relative to the work which is indicated in broken lines;

Fig. 11 is a cross section as indicated by the arrows I i in Fig. 10 showing the depending portion of the yoke, the dolly, the pressure pad extensions, toggle links and pivot pins; and

Fig. 12 is a longitudinal section looking upward as indicated by the arrows [2 in Fig. 11, showing the riveter of Fig. 1G in relation to the work (in broken lines) at the completion of the riveting operation.

Referring to Fig. 1, the material operated upon by the present riveter comprises a rivet It extending horizontally through the flanged ends of two adjacent roof sheets l5 and through a seam cap it of generally inverted U shape which snugly embraces said flanged ends. The seam cap [6 extends transversely of the roof of the car, at a slight inclination to the horizontal, and near the center of the roof it serves as a support for the running board bracket IT. The riveter of this invention is designed to press the front and rear folds of the seam cap into tight contact with the roof sheet flanges and then head the rivet with a squeezing action. The head of the rivet i l may engage the seam cap directly or may be separated therefrom by a flange of bracket ll as shown, depending upon the location of the riveting operation. The present invention, therefore, is designed to permit variations in the length of the rivet and in the total thickness of the work pieces.

A yoke 18 is arranged to house or support the various elements of the riveter. Yoke I8 is preferably in the form of a steel casting which is generally hollow, the walls of the yoke being of such thickness as to provide minimum weight and maximum strength, in accordance with well recognized principles of design. The bay it or" the yoke i8 is arranged to provide sufiicient clearance for the work, and permit the riveter to be moved both swivelly and longitudinally relative to bracket 17.

Within the upper portion of the yoke 58 is an open-ended cylinder iii which is positionally maintained by means or a press fit to circumferential projections 22 and 23 formed on the inner walls of the yoke is. A shoulder as on the exterior of the cylinder 2! in the region of the open end abuts against projection .23 to restrict forward movement of the cylinder i within the yoke 18. Threadably affixed to the open end of the cylinder 25 is a cylinder head 25 in which is arranged a passageway 25, one end of which opens into the cylinder 2! the other end of which is threaded for the reception of a pipe coupling 21, which is arranged to project into an opening 28 in the top wall of the yoke 58. Th cylinderhead 25 is locked in position by means of two set screws 29 which pass through opposite walls or the yoke i8 and engage the cylinder head as shown. The closed end of the cylinder 2! has an opening 3! which is threaded to receive a street elbow 32 in which is screwed a grip coupling 33, which passes through an opening 3 in the top wall of the yoke It, the grip coupling 33 being approximately in line with the coupling 31. Within the cylinder 25 is slidahly arranged a piston 35 which is secured, by means of a ball connection 38, to a piston rod 3! which passes through a smooth fitting bore 38 in the cylinder head 25.

The end of the piston rod 3'5 which projects from the cylinder 2i beyond the cylinder head 25, is enlarged and bifurcated by means or" vertical slot 39 (Fig. 5) which receives the upper end of a rocker arm il enclosed within the yoke !3. n opposite sides of the slot as are located two vertical grooves 52 which are arranged to receive the ends of a pin :23, retained in the upper end of the rocker arm M. It is to be noted that 'width of the grooves s2 is somewhat greater than the diameter of the pin Q3, and that the pin engages the rear side of the groove. The pin serves to transmit return stroke movement or" the piston 35 to the rocker arm, and such a groove and pin arrangement insures that no strain is caused the pin '53 during power stroke movement of the rocker arm ii. The upper end oi the rocker arm cl has a curved projection which abuts against the bottom of slot 39 on the piston rod 32' during the power stroke thereof.

The rocker arni ii pivots about its central tion by means of a transverse pin each end of which is maintained in needle bearings :35, which are surrounded by a bushing ii, the bushing being press-fitted to a bore 3 formed in wall of the yoke 58. At each end of the pin is a plate is which is secured by a screw 5i to the end of the pin i5 and which restricts lateral movement of the needle bearings 66, the plate fitting within the bore d8 as shown.

The lower end of the rocker arm ii has a curved projection 52 (Figs. 1 and 2) which ahuts against a ram or rivet set holder 53 slidably arranged in a bushing 5G (Fig. 7) which is pressfitted to a bore in the rear jaw of the yoke 8. The end of the holder 53 which engages the rocker arm l! is bifurcated, and two vertical grooves 55 are arranged on opposite inner surfaces of the bifurcated end, to receive the ends of a pin retained in the lower end of the rocker arm i. The pin 56 and groove 55 arrangement rres the purpose of transmitting return stroke movement from the rocker arm ii to the holder 53, and is similar to the arrangement of pin dB in grooves so that no strain is caused pin 56 during power stroke movement of rocker arm 6i. The axis of pivot pin it is closer to the axis of ram 53 than to the axis of power piston 35 in order to multiply the force of the piston in a well known manner. A rivet set 51' is snugly fitted to a bore in the holder 53, the rivet set 5'5 being conveniently removed when worn or when different size and style rivets are to be set, by means of a shoulder (Fig. 1) formed on the external periphery of the rivet set and against which can be driven a wedge to force the rivet set 5? from the holder 53.

On the forward jaw of the yoke i8 and opposite the rivet set 5?, is a pressure pad arrangement, which is best seen in Fig. 6. The pressure pad arrangement comprises a pressure pad 59 (Fig. 3) and a spring actuated toggle lever system consisting of a pair of front links 6i, a pair of rear links 62, a pro-loaded tension spring 63, and adjustment screws cs (Figs. 6 and 9) which pass through the links 8! and are threadably associated therewith. The front links {H are connected by pins 55 to the pressure pad 59 at one end, and each has an open end slot 56 at the other end which engages a pin 6? in one end of the rear links 52, the other end of the links 62 being connected by pins 58 to cars on the yoke is, as shown. A dolly 69, one end of which is snugly fitted to a bore H in the forward jaw of the yoke i8 and locked therein by set screw 32, extends from the yoke [8 along a center line co inciding with the center line of the rivet set 53 in the opposite jaw of the yoke 58. The pressure pad has a pair of parallel sided extensions it which are arranged to slidingly engage fiat lateral surfaces or the yoke jaw, and a centrally positioned bore i which serves for the reception of dolly 653. The pressure pad 5c is thus arra god for reciprocal motion to and from the rear jaw in a fixed plane.

Horizontal grooves in in the sides of the forward yoke jaw receive the ends of screws t as shown in Fig. 9, thus assisting in holding the pressure pad and toggle assembly in proper alinement. The spring 53 passes through a r cess 15 formed the bottom of the yoke jaw, the spring connecting at each end with a rear link $2 to urge the links toward each other.

It is to be noted that the three pins 65, 51, E58, used for connecting one side of the pressure pad arrangement are not in a straight line, the center pin El being at the greatest distance from the center oi-the yoke iii. Thus when the pressu e pad 55 is forced rearwardly in the direction of the spring 63, each of the two rear links 52 pivot about the fixed pins 68 and swing out, thereby further tensioning the spring E3. The amount of force required to initially overcome the resistance or" the spring 83 depends upon the distance of the center pin 6? from a line drawn throughthe center of the other two pins 85 and 63. The greater this distance, the greater will be the mechanical advantage and the less will be the force required to move the pressure pad 5. from the center of theyoke to obtain thev desired initial pressure of the pressure pad. Each adjustment screw as is provided with. a lock nut 11 to maintain the setting of the adjustment screw. It is apparent that the pressure pad arrangement is soarranged as to provide maximum clamping pressure of the work prior to and at the commencement of the riveting operations, followed by an automatic decreasing in pressure so that at the time the rivet upsetting begins, most of the power of the work cylinder which was applied to overcome the resistance of the pressure pad, is applied to upsetting the rivet.

Afiixed to the rear end of the yoke l8 by screws 78 is a cover l9 (Fig. 1), which preferably is in the form of a casting, and which incorporates a handle portion 8! and a switch box cavity 82. Within the switch box cavity 82 is secured an electrical switch 83 of the plunger type and having single throw normally open contacts. An electrical conduit 84, connected to the switch 83, passes upwards through the top wall of cavity 82'. A cover plate 85 encloses the switch box cavity 82 and is positionally retained by screws 86 as shown in Fig. 1. l rojecting from the lower portion of the cavity is a trigger 3'! having a shoulder 88 which rests upon a circumferential ledge 89 located in a hole extending downwardly from the cavity 3-2. A spring 9! bears against the shoulder 88 of the trigger 81 to maintain the trigger in oh: or non-operative position; however, the trigger can be manually cocked so that the shoulder will rise tofengage the plunger oi the switch 83 and thereby close the switch contacts. A trigger guard in the form of a projection 92 is arranged on the inner surface of the handle 8'i just below the trigger iii, to isolate the trigger and impede accidental manipulation thereof.

The riveter is suspended by a bail t3 which is connected at one end to the front of the yoke l8 by means of a trunnion arrangement 94. The trunnion arrangement 9% comprises a hub 95, a sleeve 86 which surrounds the hub, and a plurality of ball bearings 9i which are maintained in a pair of complementary raceways in the hub and sleeve respectively. The bail 93 is secured to the sleeve 96 by suitable means such as welding for example. A cap screw 98 tightly screws the hub to a boss 98 located on the front end of the yoke i Projecting from the trunnion arrangement 9!, and extending about the top of the boss at is an arm it?! which operatively abuts against either of two screws 92 in the boss 99, thus serving as stop to limit the amount of swivel movement of the trunnion. The bail 93 extends in a direction over the riveter and terminates at a point above the center of gravity thereof. A transverse bore 893 is provided at the free end of the ball, the bore forming an eyelet adapted to receive the hook of a balancer or hoist (not shown). It is thus seen that the riveter is suspended in such manner that it may readily be moved from one position to another and may swivel to accommodate itself itself to the different angles of the car roof.

The riveter is hydraulically operated by a standard two-way hydraulic power plant (not shown). The couplings 2i and 33 are connected by here means, not shown, to the power plant. Conduit 35-, which is connected at one end to the switch Z33, connects at the other end to electrical control means of the power plant, the electrical control means being so arranged as to initiate the hydraulic cycle of the power plant. When the trigger 81 i operated to close the electrical contacts of switch 83:, the electrical control means of the power plant respond to set into operation a pumping cycle whereby high pressure oil. is supplied to the high. pressure hose connected to coupling 33, thereby forcing. the piston 35 rearward in cylinder 2i, thus commencing a rivet setting operation. After the rivet has been headed and a predetermined pressure is reached within the cylinder, the power plant automatically reverses its cycle (in response to the back pressure of oil in the high pressure hose connected to coupling 33) and pumps low pressure oil through the hose connected to coupling 21 whi e exhausting the high pressure hose, until the piston 35 has returned to first position, after which the operation of the power plant automatically ceases. Upon release of the trigger 81 and the reactuation thereof, the power plant repeats the cycle as above set forth.

In operation the rivetcr shown in Figs. 1-9, is positioned so that the rivet set 5! contacts the head of the rivet which is located in a hole in the work piece, the other end of the rivet extending through the work piece in the direction of the pressure pad 59 as shown in Fig. 1. The trigger Si is pressed initiating operation of the power plant and resulting in pressurized oil being delivered to the cylinder from the hose connected to coupling 33. The piston 35 is forced rearwardly in the cylinder 2 l causing the rocker arm ii to be rotated clockwise about the pin 45 thereby tending to drive the rivet set holder 53 forward, or in the direction of the opposite jaw of yoke I8. However, since the holder 53 abuts against the work piece by way of rivet set 5?, the work piece being stationary prevents forward movement of the rivet set holder 53, with the result that the motion is transferred to the yoke l8, which thereupon moves in a rearward direction in unison with the pressure pad 59 causing the latter to engage the work piece, or the seam cap assembly as is illustrated. Continued movement of the yoke I 8 causes the pressure pad 59 to tightly squeeze the work piece until the dolly '69 moves rearward relative to the pressure pad and contacts the rivet. The operator then observes through the transverse slot H84 in the pressure pad 59 whether or not the dolly 59 and rivet M are properly alined, releasing the trigger 8? if necessary to prevent misoperation. If the trigger is kept pressed, the oil pressure and mechanical pressure build up rapidly and the rivet heading operation begins, but the pressure of the pad 59 is gradually diminished as the mechanical advantage the link toggle system increases, in the manner as heretofore explained.

When the rivet heading is completed and the piston 35 has travelled its maximum distance, the pressure in the high pressure deliverv hose builds up to such a value as to automatically reverse the operating cycle of the power plant, whereupon the oil in the high pressure hose is released and low pressure oil is delivered to the cylinder 2| by way of the hose connected to the coupling 2'11, thereby forcing the piston to return to its first position in the cylinder. In so doing, the rocker arm 4| is moved in counter clockwise direction thus causing the return of the rivet set holder 53 and the pressure pad arrangement to initial position so that the riveter can be removed for positioning for subsequent riveting operations.

Referring now to Figs. 10, ll and 12, the modified riveter comprises a yoke K35 having a front jaw liifiand a rear jaw ltl. The rear jaw has a longitudinal bore I08 receiving with a press fit a cylinder liner I09 and has a counterbore I I I surrounding an enlarged diameter portion of the liner. A two-diameter ram or rivet set holder II 2 is slidably fitted to the liner. Any suitable means may be provided for actuating the ram forwardly relative to the cylinder liner on its power stroke and for returning it, for example a rocker arm I I3 and pin II I connection similar to the arrangement of Figs. 1 and 2. The front end of the ram is bored to receive a detachable rivet set I I5 engageable with the head of a rivet I I6 mounted in a set of sheets I I1.

Engageable with the shank end of the rivet H6 is the rear extremity of a dolly III} whose front end fits within a bore III; in yoke jaw its, being retained by set screw I2I. Dolly IIEI has a sliding fit in a bore I22 in the body portion of a pressure pad I23. The pressure pad is arranged to press the work pieces II'I tightly against the rivet set II5 in the manner of the pressure pad 59 shown in Fig. 3. It has an upper pair of flanges I24 projecting forwardly from the body portion, the inner edges of the fingers slidably fitting the Sides of a depending portion of front jaw I06. It has a lower pair of fingers I 25 of greater width than the upper pair, similar- 1y engaging a narrower part of the jaw near the lower extremity of the latter. The jaw Iu'lii has a pair of ribs I26 extending between the upper and lower fingers and fitting the latter to provide a track which guides the pressure pad in its sliding movement relative to the yoke. Extending vertically on opposite sides of the yoke jaw I525 is a pair of pins I2'I each supported by pressure pad fingers I24 and I25 and providing a pivot for the rear end of rear link I28. The front end of each link I23 is pivotally connected by vertical pin I29 to a front link ISI. The front links in turn are pivoted, by pins I32, to the front jaw I06 on yoke I85. A tension spring I33 connects the two front links and tends to pull the intermediate pivot pins I29 toward each other at all times. In the normal or non-operative condition of the parts the inward collapsing movement of the links is limited by a screw I34 extending transversely through one of the rear links I28, the inner end of the screw abutting against the associated rib I26 on yoke jaw I36. The links I28 and I3I cooperate with the spring I33 and screw I34 to provide a toggle connection between the pressure pad I23 and yoke I55 operating in a similar manner to the toggle of Fig. 5.

In operation, the entire riveting tool is moved until the rivet set H5 engages the head. of rivet IIB. The power is turned on, and the rocker arm H3 is actuated to urge the ram IIE forward and the yoke I05 rearward. Since the is held against movement by the rivet Iii and stationary work pieces III, the entire yoke I85 moves back carrying with it the front jaw its and the dolly H8 and also during the initial movement of the yoke, the pressure pad I23. The latter comes 'into contact with the sheets III pressing them firmly against the head of rivet H6, which is backed up by rivet set I I5. Thereafter, continued movement of the yoke I55 in rearward direction is accompanied by continued movement of the dolly IIS but not the pressure pad I23. The relative movement between the yoke and pressure pad spreads the pivot pins apart, as above described in connection with Fig. 6, and gradually relieves the yoke of the opposition to movement offered by the pressure pad and. toggle assembly, while the component of force transmitted through the dolly increases. Continued movement of the dolly I I8 relative to the pressure pad I23 heads the rivet until the parts finally come to rest as shown in Fig. 12.

It is thus apparent that the power stroke of the riveter includes four stages: first, the unopposed movement in unison of yoke jaw IE5, dolly I I8 and pressure pad I23, which stage terminates when pressure pad contacts work sheets I II; second, the movement of the pressure pad (substantially in unison with the yoke and dolly) for a slight distance to compress the work sheets, which stage ends with the pressure pad coming to rest; third, the movement of the yoke and dolly relative to the pressure pad, under the decreasing force of opposition of the toggle alone, which stage ends when the dolly contacts the rivet; and fourth, the movement of the yoke and dolly against the resistance of the rivet being headed and also against the (further decreasing) resistance of the toggle.

The set screw I3 3 should be adjusted to permit the pressure pad I23 to firmly engage the work sheets II'I just before the dolly II8 engages the shank end of rivet H6, or in other words to minimize the length Of stroke in the third stage, by making the first, or unopposed stage continue as long as possible. If the screw is tightened too much, that is, if it is projected too far inwardly, it will hold the pressure pad I23 out of contact with the work pieces IIT until after the dolly IIS and rivet set H5 have started to head the rivet and thus fail to insure a tight connection; or in other words, it will delay the second stage and make it overlap the fourth. If the screw is too loose it will shortenthe first stage and cause the pressure pad to start functioning sooner than necessary, thereby resulting in a waste of power and increased wear and breakage as the spring I33 is repeatedly tensioned and untensioned.

The setting of the screw I33 may be retained for successive riveting operations as long as there is no variation in the structure of the workpieces or the distance that the shank end of the rivet I Iii projects beyond the work sheets I II. Variations caused by the successive inclusion and omission of a running board bracket I! (Fig. 1), may be compensated for by the use of rivets Of corresponding length, without requiring a resetting of adjusting screw. On the other hand, an in crease in the thickness of the sheet metal which forms the seam cap would call for a change in the adjustment of the screw because it would require a greater initial clamping pressure to squeeze the assembly together prior to riveting. In that case, the set screw Its is loosened to permit the intermediate pins I29 to come somewhat closer together and the pressure pad I23 to shift slightly further to the right as compared with Fig. 10. When the power is applied there will be a slightly shorter stroke than usual in the first stage resulting in a correspondingly longer stroke in the third stage. At the time the fourth stage begins the pressure on pad I23 will have diminished considerably below the maximum but will still be effective to hold the workpieces in snug engagement. A slight lengthening or shortening of the stroke in the first stage caused by change in the setting of screw I3 results in a very great change in the initial mechanical advance of the toggle and the initial (but not the final) pressure on the pad I23.

What is claimed is:

1. A pressure pad arrangement comprising a pad support, a pressure pad element having a 9 pair of parallel sided extensions which slidingly engage the sides of the pad support, a pair of rear links symmetrically disposed about said pad support, the rear end of said links being pivotally connected to said pad support, a pair of front links each of which interconnects a rear link and a parallel sided extension of said pressure pad element, and a preloaded tension spring, each end of which connects to a rear link to draw said rear links together.

2. A pressure pad arrangement according to claim 1, means to adjust the amount of preloading of said spring, said means comprising a screw which is threadably associated with a front link to adjustably project therefrom and abut against the side of said support, a lock nut on the adjustment screw and arranged so that it can be turned into abutment against the side of the link.

3. A pressure pad arrangement comprisin a pad support, a pressure pad element adapted to engage the work pieces slidably connected to the pad support, said pressure pad element having a through opening, said pad support adapted to carry a dolly to extend through the through opening of the pressure pad element, a pair of rear links symmetrically disposed about the pad support, and the rear ends of said links being pivotally connected to the pad support, a pair of front links and each of which is pivotally connected to a rear link and to the pressure pad element, and a preloaded tension spring connected between the said rear links to draw the links together.

4. A pressure pad arrangement comprising a pad support, a pressure pad element adapted to engage the work pieces slidably connected to the pad support, said pressure pad element having a through opening, said pad support adapted to carry a dolly to extend through the through opening of the pressure pad element, a pair of rear links symmetrically disposed about the pad support, and the rear ends of said links being pivotally connected to the pad support, a pair of front links and each of which is pivotally connected to a rear link and to the pressure pad element, and a preloaded tension spring connected between the said rear links to draw the links together, said pad support having grooves in the 10 carry a dolly to extend through the through opening of the pressure pad element, a pair of rear links symmetrically disposed about the pad support, and the rear ends of said links being pivot-ally connected to the pad support, a pair of front links and each of which is pivotally connected to a rear link and to the pressure pad element, and a preloaded tension spring connected between the said rear links to draw the links together, said pressure pad element having a pair of spaced parallel sided extensions lying on the opposite sides of the pad support, said pad support having ribs lying respectively at th opposite sides thereof, said parallel extensions of the pressure pad element lying respectively on the opposit sides of a rib whereby to be guided by the same.

6. A pressure pad arrangement comprising a pad support, a pressure pad element adapted to engage the work pieces slidably connected to the pad support, said pressure pad element having a through opening, said pad support adapted to carry a dolly to extend through the through opening of the pressure pad element, a pair of rear links symmetrically disposed about the pad support, and the rear ends of said links being pivotally connected to the pad support, a pair of front links and each of which is pivotally connected to a rear link and to the pressure pad element, and a preloaded tension spring connected between the said rear links to draw the links together, said pressure pad element having a pair of spaced parallel sided extensions lying on the opposite sides of the pad support, said pad support having ribs lying respectively at the opposite sides thereof, said parallel extensions of the pressure pad element lying respectively on the opposite sides of a rib whereby to be guided by the same, said front links being pivotally connected with the parallel sided extensions, each link lying between the respective side extension of a pair, and an adjusting screw adjustable in one of the front links and engageable with a rib whereby t0 predetermine the amount of loading of the tension spring.

GEORGE T. HARCOURT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Kaman Oct. 23, 1945 

